I googled this up:
http://www.chipsplace.com/helpful/Airbus/Hydraulics.htmIt's about A320 but I believe the hydraulics are quite similar.
"Green system – 1 pump: engine driven. Two power sources: engine 1 pump & PTU
Blue system – 2 pumps: 1 electric and the emergency RAT. Two sources of power: electric pump & RAT pump.
Yellow system – 3 pumps: 1 engine, 1 electric & 1 hand pump. 4 sources of power: engine 2 pump, electric pump, hand pump and PTU.
Green is the “heavy” system with landing gear, flaps/slats, N/W STRG and Normal Brakes.
Blue is basically for redundancy with the only unique items on it being L & R spoiler 3 and the Emergency Generator which are “backup” items themselves.
The alternate brakes are powered by the Yellow hydraulic system and will automatically become selected if Green hydraulic is insufficient for normal brakes. Yellow brakes have the same capabilities as normal brakes except for autobrake capability."
So it appears that the brakes are powered by 2 of the hydraulic systems.
I know I have too much time on my hands...